1. Field of the Invention
The present invention relates to a color image forming apparatus and method and an image processing program, which are applicable to a color print system that prints using color materials consisting of general process colors (cyan, magenta, yellow and black).
2. Description of the Related Art
A color print system using general process colors (cyan, magenta, yellow and black) forms full-color images by forming plates of individual colors using color materials and by combining the plates according to a subtractive mixture theory.
When forming a full-color image, unless the individual color plates are registered precisely, a blank space, generally called a registration gap, can occur between the plates, between image portions that ought to be adjacent to each other exactly. FIG. 5 is a conceptual illustration with reference to which the phenomenon will be described. FIG. 5 shows an example in which a letter image of a cyan plate is superimposed on a uniform background image of a magenta plate. The magenta plate has an image, from which the letter portion of the cyan plate is omitted. This is because mixing the magenta with cyan will result in blue. To represent the letters in cyan, therefore, the magenta plate has the portions corresponding to the letters omitted.
When the magenta plate and the cyan plate have their superimposed positions unmatched, the background color is seen around the letters of the cyan plate as shown in FIG. 5, which is called a registration gap.
Conventionally, to eliminate the registration gap, a technique called trapping processing is applied to image portions in which the plates of individual colors are adjacent to each other. FIG. 6 is a conceptual illustration with reference to which the trapping processing method will be described. The trapping processing method reduces the area of the hollow portions of the magenta plate constituting the background so that the contours of the letter portions of the cyan plate are superimposed on the magenta plate with some width (the “trap width”) left in between.
Trapping processing can prevent the occurrence of a registration gap even if the superimposed positions of the magenta plate and cyan plate are displaced from each other as long as the displacement is within the trap width.
A concrete example of trapping processing is as follows. Specifically, an image designer can set an optimum trap width for each object, thereby being able to reflect it on a print image. There is a printer system with a function that applies a preset trap width to all portions in an image. When the printer system automatically carries out the trapping processing, the trapping processing is applied to portions that have no overlap margins (overlap widths/spaces) between plates of different colors. When no overlap margins are present between the plates of different colors, the drawing regions of the lower color plate are revised to provide the margins between the lower and upper color plates.
According to the conventional technique, it is necessary in trapping processing to adjust the trap width manually (see, Japanese Patent Application Lid-Open No. 2004-262011, for example), and otherwise only one value is applied to the entire image uniformly.
Such manual adjustment has a problem of being inefficient because the designer of the image data must carry out the trapping processing manually from image to image using application software capable of trapping processing setting.
When the printer system side uses the automatic trapping processing, the preset trap width is applied to all portions having no overlap margins between the color plates of different colors within the image. As a result, it has an adverse effect in that the trap width can become too large or too small in part. More specifically, when the cyan plate is adjacent to the magenta plate, since the superimposed portions become red because of mixing, too large a margin will result in an image of a cyan object with red edges. In contrast, too small a margin has the problem of being unable to absorb the entire registration gap.